hwmon: (core) Deprecate hwmon_device_register()

Inform the user that hwmon_device_register() is deprecated,
and suggest conversion to the newest API. Also remove
hwmon_device_register() from the kernel API documentation.

Note that hwmon_device_register() is not marked as __deprecated()
since doing so might result in build errors.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>